MOLLUSCOUS ANIMALS. 



397 



teriorly to each other, and to the mantle; sexes distinct; inhabit fresh 

 water or estuaries ; shell generally nacreous within. Families — 



1 MuUeridss. 



2 ^theriadae. 



3 Anodontidae. ' 



4 Trigoniadae. 



5 Unionidae. 



Ord. III., Beachysiphonid^. — Animal with Siphons, the mantle 

 lobes being more or less united ; Si]Dhons short, pallial line simple. 



r ChaminEe, 

 Families : 1 Chamidas, with sub-families } Hippuritidinse, 



(^ TridacniuEe, 



2 Lucinidae. 



3 Cardiidas. 

 4: Cycladidae. 

 5 Cyprinidse. 



Ord. IV., Macrosiphonida. — Animal with long retractile Siphons ; 

 Pallial line Sinuated. 



Families : 1 Myadas, including Anatinidas auct. 



2 Solenidffi. 



3 Mactridse. 



4 Tellinidie. 



5 Veneridfe. 



Ord. V. Inclusa. — Animals boring in stone, clay or wood, their long 

 imperfectly retractile siphons sometimes inclosed in a shelly tube, to 

 which the pair of shells is sometimes attached ; shell incompletely cover- 

 ing the body, often hard and rasp-like, and with accessory pieces about 

 the umbones. 



Families not well determined — Glastrochaenidae and Pholadidse are 

 generally recognised. 



I have not here attempted to give the characters of the families 

 adopted, because they may be found in accessible books, except in the 

 case of the Naides, where in giving them the importance of a distinct 

 order, to which I think them well entitled, I have associated with 

 them the little family of Trigoniada3, agreeing with them in the large 

 active foot, in the nacreous interior of the shell, and in having, as it 

 appears to me, their curious hinge teeth, more related to those of Unio- 

 nidae than to any other form. I have also ranked as a family distinct 

 from Unionidae, the shells without hinge teeth forming the genera Ano- 

 don and Mycetopus, with which Iridina must be associated, and I have 



